Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well-suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of glass, metal, or plastic containers, used for holding cosmetic, pharmaceutical, liquor, and health care products.
Reviews of various aspects of ink jet printing can be found these publications: Kuhn et al., Scientific American, Apr. 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in the following U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153, and 3,673,601.
In general, an ink jet ink composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Further, the ink must be quick-drying, smear resistant, and be capable of passing through the ink jet nozzle without clogging, and permit rapid cleanup of the machine components with minimum effort.
The marking of articles such as bank checks, envelopes, certificates, and the like, as well as food containers such as metal, plastic or glass containers with identification marks for later identification and/or sorting is well known. Several methods have been proposed for producing such security or identification marks. For example, infrared readable bar codes have been proposed by the Japanese Patent Application Kokai No. 58-45999 to meet the need for invisible security marks. Thus, a voting card is printed with an infrared absorbing bar code, and the printed area is covered by smearing with an ink which has high light absorptivity in the visible region but low light absorptivity in the infrared region. This method has the problem of producing disagreeable visual images because of the black color of the ink which has low light absorptivity in the infrared region. This method also has the problem that it is incapable of perfectly concealing the bar code in order that it would be invisible to the unaided eye.
U.S. Pat. No. 5,366,252 discloses another method of concealing the infrared absorbing bar code. An infrared absorbing mark formed by printing on a substrate is covered up by smearing with an ink having high light absorptivity in the visible region and pervious to light in the infrared region, thus forming a colored concealing layer on the mark, and on this colored concealing layer there is further provided a white concealing layer containing a white pigment and an extender pigment of which at least part of the particles have an average size of 3 to 20 microns.
The aforesaid methods have the disadvantage that the infrared absorbing bar codes are to some extent visible to the unaided eye and need to be physically concealed. The concealment of the bar code results in covering up of a portion of the article, thereby adversely affecting the aesthetics of the article.
Fluorescent materials have been considered for marking purposes. It is known that fluorescence is the property of a material to emit radiation as the result of exposure to radiation from some other source. The emitted radiation persists only as long as the exposure is subjected to radiation. The fluorescent radiation generally has a longer wavelength than that of the absorbed radiation.
There has been significant developmental activity in the area of fluorescent jet inks for producing security marks on envelopes and documents. For instance, U.S. Pat. No. 5,093,147 discloses a method for providing intelligible marks that are virtually invisible to the unaided eye on the surface of an article. The invention is based on a jet ink containing an organic laser dye that is poorly absorptive in the visible range of about 400 to 700 nm, is absorptive of radiation in the near infrared range of at least 750 nm, and fluoresces in response to radiation excitation in the infrared range at a wavelength longer than that of the exciting radiation.
U.S. Pat. No. 4,736,425 discloses a method of marking fiduciary documents requiring authentication by the use of certain fluorescent chelates. The method comprises introducing only a part of the elements forming the chelate onto the document to be marked and subsequently contacting the document for authentication purpose with the missing part of the elements forming the chelate to effect the synthesis of the fluorescent chelate. The chelate thus formed is excited by ultraviolet radiation and the resulting fluorescence radiation is detected.
U.S. Pat. No. 4,450,595 discloses a jet ink that can be used to mark documents such as bank checks for automatic identification. The ink contains certain phenoxazine derivative dyes that are visible to the unaided eye and fluoresce in the near infrared region (650 to 800 nm) upon activation using an activating light having a wavelength in the range of 550 to 700 nm. The ink that is visible to the unaided eye is unfortunately not suitable for many security mark applications.
Commonly owned and copending U.S. patent application Ser. No. 08/661,180, filed Jun. 10, 1996, discloses jet ink compositions suitable for mark on white or light colored substrates such as envelopes. The ink composition comprises a fluorescent colorant and an ink carrier. The mark produced by the ink composition is completely or substantially invisible to the unaided eye and is visible only when excited by ultraviolet light.
In the area of marking objects such as metals, the following publications are of interest. German Patent DE 3529798 reportedly discloses a jet ink for placing on metals, plastics, paper or glass identification marks that are invisible to the naked eye consisting of an alcohol solvent, a fluorescent substance that is soluble in a water/ethanol mixture, a water-soluble polyacrylate, and optionally a water-soluble cellulose ester and diethanolamine.
German Patent DE 4013456 reportedly discloses a jet ink containing an organic solvent, a fluorescent dyestuff, a polyamic acid or polyimide binder resin, and conductive salts. The ink is said to adhere well to glass ceramic and copper.
The foregoing indicates that there exists a need for a jet ink composition comprising a fluorescent colorant suitable for printing identification marks on metals, glass, ceramics, and plastics.
Metal containers such as, for example, empty containers used to bottle air fresheners, mosquito repellants, shaving creams, carpet cleaners, oven cleaners, automobile body shine boosters, primers, and the like, are shipped to the fillers with identification marks placed thereon by the container manufacturer. At the fillers' premises, the containers are subjected to leak testing by immersing the filled containers in hot water. The leak testing typically is carried out by immersing the containers in water at 150.degree. F. for 5 minutes. It has been a problem with the previously known fluorescent jet ink compositions that the marks tend to blush when exposed to the hot water. As a result, the marks become visible to the unaided eye, and thus easily readable by the filler. Manufacturers would prefer to keep the marks, which may contain proprietary information, confidential.
Thus, there exists a need for a jet ink composition suitable for printing on containers marks that do not blush when exposed to warm and humid conditions, particularly those encountered during the leak testing of filled containers.
Furthermore, during the manufacture of metal containers, the containers are exposed to high temperatures to effect the curing of the coatings applied to the containers. The containers may be exposed to temperatures in the range of from about 350.degree. F. to about 700.degree. F. Typically, the containers are exposed up to about 390.degree. F. for about 30 seconds. The identification marks should be able to resist the heat exposure. Thus, there exists a need for a jet ink composition that produces a mark which resists such a heat exposure.
These and other objects of the present invention will be apparent from the detailed description of the preferred embodiments of the invention set forth below.